Foldable display and portable terminal device

ABSTRACT

Provided is a foldable display that is excellent in mass production applicability and unlikely to have image distortion at the folding portion of the display after repeated folding, as well as a mobile device equipped therewith. The foldable display contains a surface protection film and a polarizer. The surface protection film is composed of a polyester film, and the angle made by the slow axis of the polyester film with the absorption axis of the polarizer is 10 to 80°. The polyester film has a refractive index in the bending direction of 1.590 to 1.620, a refractive index in the direction of a folding portion of 1.670 to 1.700, a refractive index in the thickness direction of 1.520 or less, and a density of 1.380 g/cm3 or more, wherein the bending direction refers to a direction orthogonal to the folding portion of the polyester film to be folded.

TECHNICAL FIELD

The present invention relates to foldable displays and mobile devices.The present invention further relates to foldable displays and mobiledevices that are unlikely to have image distortion caused by deformationof the film even when they are repeatedly folded.

BACKGROUND ART

Becoming thinner and lighter, mobile devices such as smart phones havebecome widely prevalent. While being required to have a variety offunctions, mobile devices are also required to be convenient to use. Itis taken for granted that prevailing mobile devices can be operated withone hand for simple tasks, and can be put into a clothes pocket. Thus,such mobile devices must have a small screen size, such as about 6inches.

Tablet devices with a screen size of 7 to 10 inches are intended for usenot only for video content and music, but also for business purposes,drawing, and reading, and thus have a high level of functionality.However, these devices cannot be operated with one hand and are notparticularly portable, leaving some issues in regards to convenience.

In order to meet the above requirements, PTL 1 suggests a technique ofmaking compact devices by connecting multiple displays. However, due tothe remaining bezel portion, the image is split, and visibilitydecreases. Thus, this technique is not commonly used.

More recently, mobile devices equipped with a flexible or foldabledisplay have been proposed. This technique enables conveniently carryinga mobile device equipped with a large screen display without problems ofimage splitting.

Conventional displays and mobile devices that have no folding structurecan be protected by a non-flexible material, such as glass, that isapplied to the surface of the display. However, a foldable display thatuses a single screen spanning over a folding portion must be protectedby, for example, a flexible and surface-protecting hard coating film.However, a foldable display is repeatedly folded at the point where aportion folds, and the film at that point deforms over time, causingimage distortion on the display. In addition to the surface protectionfilm, films are further used in various parts of a foldable display,such as a polarizing plate, a retardation film, a touchscreen substrate,a substrate of display cells such as organic EL, and protectivematerials on the back. These films are also required to be durableagainst repeated folding.

PTL 2 suggests a technique of partially altering the film thickness.However, this method is not suitable for mass production.

A technique to adjust the refractive index of a polyester film in thebending direction is also suggested (see PTL 3). However, the pencilhardness during the application of hard coating decreases with adecrease in the refractive index in the bending direction, thus loweringthe surface protection functionality of the display. Additionally, whiledecreasing the refractive index in one direction reduces deformationthat occurs when the display is folded, it makes the folding directionmore uniaxially oriented, forming cracks or breaking the display at thefolding portion.

CITATION LIST Patent Literature

-   PTL 1: JP2010-228391A-   PTL 2: JP2016-155124A-   PTL 3: W02018/150940A

SUMMARY OF INVENTION Technical Problem

The present invention is intended to solve the problems that arise inthe parts of conventional displays as described above. The invention isintended to provide a foldable display that is suitable for massproduction and that is unlikely to have distortion of images on thefolding portion after the display is repeatedly folded, and to provide amobile device equipped with such a foldable display.

Specifically, the present invention includes the following subjectmatter.

-   Item 1. A foldable display comprising a surface protection film and    a polarizer,-   wherein

the surface protection film comprises a polyester film,

the angle made by the slow axis of the polyester film with theabsorption axis of the polarizer is 10 to 80°, and

the polyester film satisfies the following conditions:

-   (1) the polyester film has a refractive index in the bending    direction of 1.590 to 1.620,-   (2) the polyester film has a refractive index in the direction of a    folding portion of 1.670 to 1.700,-   (3) the polyester film has a refractive index in the thickness    direction of 1.520 or less, and-   (4) the polyester film has a density of 1.380 g/cm³ or more, wherein    the bending direction refers to a direction orthogonal to the    folding portion of the polyester film to be folded.-   Item 2. The foldable display according to Item 1, wherein the    polyester film has an in-plane retardation (Re) of 3000 to 30000 nm.-   Item 3. The foldable display according to Item 1 or 2, wherein the    polyester film has a total light transmittance of 85% or more, a    haze of 3% or less, and a maximum heat shrinkage of 6% or less.-   Item 4. The foldable display according to any one of Items 1 to 3,    comprising an easy-to-adhere layer on at least one surface of the    polyester film.-   Item 5. The foldable display according to any one of Items 1 to 4,    comprising a hard coating layer having a thickness of 1 to 50 μm on    at least one surface of the polyester film.-   Item 6. A mobile device comprising the foldable display of any one    of Items 1 to 5.

Advantageous Effects of Invention

While the foldable display using a polyester film or a hard coating filmfor foldable displays according to the present invention maintains itssuitability in mass production, the polyester film is unlikely to havecracks at the folding portion, deform after being repeatedly folded, andhave image distortion at the folding portion of the display. A mobiledevice equipped with the above foldable display using a polyester filmor hard coating film provides beautiful images and has a variety offunctions, while being highly convenient such as in portability.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram showing the bend radius of the foldabledisplay according to the present invention being folded.

FIG. 2 is a schematic diagram showing the bending direction of thepolyester film for a foldable display according to the presentinvention.

DESCRIPTION OF EMBODIMENTS Display

The term “display” as used in the present invention refers to displaydevices in general. There are several types of displays such as LCDs,organic EL displays, inorganic EL displays, LEDs, and FEDs; however,LCDs, organic ELs, and inorganic ELs, which have a foldable structure,are preferable. In particular, organic EL displays and inorganic ELdisplays, which can reduce the layer structure, are preferable; andorganic EL displays, which have a wide color gamut, are even morepreferable.

Foldable Display

A foldable display is a single continuous display that can be folded inhalf or other ways when carried. The size of the fordable display can bereduced by half or even more by folding it, and thus the portability isimproved. The foldable display preferably has a bend radius of 5 mm orless, and more preferably 3 mm or less. A bend radius of 5 mm or lessenables the foldable display to be thin when folded. A smaller bendradius is better; however, the smaller the bend radius, the more easilycreases form. The bend radius is preferably 0.1 mm or more, or can beeven 0.5 mm or more or 1 mm or more. Even a bend radius of 1 mm or morecan achieve a reduction in thickness sufficient for practical use incarrying the display. The bend radius of a folded display refers to avalue measured at the point indicated by numerical reference 11 in theschematic diagram of FIG. 1, and is the inner radius of the foldingportion of the display being folded. The surface protection film,described later, may be positioned on the outer side or inner side ofthe foldable display.

The foldable display may be a three-fold or four-fold display, or arollable display, which is a retractable display. All of these displaysfall within the scope of the foldable display according to the presentinvention.

Foldable Organic EL Display

The essential component of a foldable organic EL display is an organicEL module. A foldable organic EL display may further optionally include,for example, a circularly polarizing plate, a touchscreen module, asurface protection film, and a back protection film.

Organic EL Module

A typical structure of an organic EL module includes an electrode, anelectron transport layer, a light-emitting layer, a hole transportlayer, and a transparent electrode. The polyester film according to thepresent invention can be used as a substrate on which an electrode isprovided, and further an electron transport layer, a light-emittinglayer, and a hole transport layer are provided. In particular, thepolyester film according to the present invention can be preferably usedas a substrate for a transparent electrode. In this case, because thesubstrate film is required to have a high level of barrier propertiesagainst water vapor or oxygen, the polyester film according to thepresent invention is preferably provided with a barrier layer such as ametal oxide layer. To enhance the barrier properties, multiple barrierlayers may be provided, or multiple polyester films provided with abarrier layer may be used.

Touch Panel Module

The mobile device preferably includes a touchscreen. An organic ELdisplay for use preferably includes a touchscreen module on the organicEL display or between the organic EL module and the circularlypolarizing plate. The touchscreen module includes a transparentsubstrate such as a film and a transparent electrode provided on thetransparent substrate. The film for use as a transparent substrate fortouchscreens is preferably provided with a hard coating layer or arefractive index adjustment layer. The transparent substrate usable intouchscreens includes polyester films, polyimide films, polyamide films,polyamide-imide films, polycarbonate films, acrylic films, triacetylcellulose films, cyclo-olefin polymer films, polyphenylene sulfidefilms, and polymethylpentene films.

Circularly Polarizing Plate

The circularly polarizing plate suppresses the deterioration of imagequality due to the reflection of external light by the components insidethe display. A circularly polarizing plate includes a linear polarizerand a retardation film. The linear polarizer preferably includes aprotection film at least on the visible side of the polarizer. Insteadof the protection film, a protective coating layer may be provided. Aprotection film or a protective coating layer may be provided on theside opposite the visible side of the polarizer, and a retardation filmmay be directly stacked on the polarizer. The retardation film for useis a resin film with a phase difference such as triacetyl cellulose,polycarbonate, or cyclic-olefin, or such a resin film with or without aphase difference provided with a retardation layer composed of a liquidcrystal compound. The protection film for polarizers includes thoselisted above as transparent substrate films for touchscreens.

Surface Protection Film

Because a shock applied to a display from above may break the circuitryof an organic EL module or a touchscreen module, a surface protectionfilm is provided in most cases. In the present invention, a polyesterfilm with specific properties is preferably used as the surfaceprotection film. The surface protection film includes a “cover window,”which is incorporated into the top surface of the display, and an “afterfilm,” which can be attached, peeled off, and replaced with another bythe user. The polyester film with specific properties is usable in bothcases, or in either case. If the polyester film with specific propertiesis used in either case, the other surface protection film for useincludes those listed above as transparent substrate films fortouchscreens. The polyester film preferably has a hard coating layerstacked at least on the front surface of the polyester film. Thepolyester film is provided on the front surface of a foldable displaywith the hard coating layer on the viewing side. The hard coating layermay be provided on both surfaces of the polyester film.

Back Protection Film

A protection film is also preferably provided on the back of thedisplay. The polyester film with specific properties used in the presentinvention can be used as the protection film for the back. Theprotection film for the back may be those listed as the transparentsubstrate film for touchscreens.

The polyester film may be a monolayered film composed of one or moretypes of polyester resins. If two or more types of polyester are used,the polyester film may be a multilayered film or ultra-multilayeredlamination film with a repeating structure.

Examples of polyester resins for use in the polyester film includepolyethylene terephthalate, polybutylene terephthalate,polyethylene-2,6-naphthalate, and polyester films composed of copolymersthat contain the component of these resins as the main component. Inparticular, from the standpoint of mechanical properties, heatresistance, transparency, and price, drawn polyethylene terephthalatefilms are particularly preferable.

When a polyester copolymer is used in a polyester film, the dicarboxylicacid component of the polyester can be, for example, aliphaticdicarboxylic acids, such as adipic acid and sebacic acid; aromaticdicarboxylic acids, such as terephthalic acid, isophthalic acid,phthalic acid, and 2,6-naphthalene dicarboxylic acid; or multifunctionalcarboxylic acids, such as trimellitic acid and pyromellitic acid. Theglycol component can be, for example, fatty acid glycols, such asethylene glycol, diethylene glycol, 1,4-butanediol, propylene glycol,and neopentyl glycol; aromatic glycols, such as p-xylene glycol;alicyclic glycols, such as 1,4-cyclohexanedimethanol; or polyethyleneglycols with an average molecular weight of 150 to 20,000. The massratio of the copolymer component of the copolymer is preferably lessthan 20 mass %. A mass ratio of less than 20 mass % is preferablebecause film strength, transparency, and heat resistance are retained.

In the production of a polyester film, at least one type of resin pelletpreferably has an intrinsic viscosity of 0.50 to 1.0 dl/g. An intrinsicviscosity of 0.50 dl/g or more is preferable because such an intrinsicviscosity increases the shock resistance of the obtained film, and thusmakes it unlikely for the internal circuit of the display to be brokenby an external shock. An intrinsic viscosity of 1.00 dl/g or less ispreferable because it prevents filtration pressure of the molten fluidfrom becoming too high, thus making it easier to stably perform filmproduction.

The polyester film preferably has a thickness of 10 to 80 μm, and morepreferably 25 to 75 μm. A thickness of 10 μm or more provides a pencilhardness improvement effect and an impact resistance improvement effect.A thickness of 80 μm or less is advantageous in weight reduction andprovides excellent flexibility, processability, and handleability.

The surface of the polyester film in the present invention may be smoothor uneven. However, a decreased level of optical properties due tounevenness is not preferable, because the film is used for covering thesurface of a display. The haze is preferably 3% or less, more preferably2% or less, and most preferably 1% or less. A haze of 3% or less canimprove the visibility of images. Although a lower haze is better, thelower limit of haze may be preferably 0.1% or more, or 0.3% or more,from the standpoint of stable production.

Given the purposes of reducing haze as described above, the film surfaceshould not be too uneven. However, to provide the film with moderateslipperiness for handleability, unevenness may be formed on the filmsurface by adding particles to the polyester resin layer of the surfacelayer, or by applying a particle-containing coating layer to thepolyester resin layer during the film-forming process.

The method for adding particles to a polyester resin layer can be aknown method. For example, particles can be added at any stage ofpolyester production and can be preferably added in the form of slurryprepared by dispersing the particles in, for example, ethylene glycol,in the esterification stage, after the completion oftransesterification, or before the start of polycondensation tofacilitate polycondensation. Alternatively, particles can be added by amethod of blending slurry prepared by dispersing particles in ethyleneglycol or water with a polyester material by using a kneading extruderequipped with a vent, or a method of blending dry particles with apolyester material by using a kneading extruder.

In particular, preferable is a method of homogeneously dispersing theaggregates of inorganic particles in a monomer solution, which is partof a polyester material, then filtering the dispersion, and adding thefiltrate to the remainder of the polyester material before, during, orafter esterification. Due to the low viscosity of the monomer solution,this method enables homogeneous dispersion of particles andhigh-precision filtration of the slurry in a simple manner, whileensuring excellent particle dispersibility and low likelihood of theoccurrence of new aggregates when particles are added to the remainderof the polyester material. From this viewpoint, it is particularlypreferable to add particles to the remainder of the polyester materialat a low temperature before esterification.

Additionally, the number of protrusions on the surface of the film canbe further reduced by a method of preparing a particle-containingpolyester beforehand, and kneading the particle-containing polyesterpellets with particle-free pellets to extrude pellets (master batchmethod).

The polyester film may contain various additives within the range inwhich the desired total light transmission is maintained. Examples ofadditives include an antistatic agent, a UV absorber, and a stabilizer.

The polyester film has a total light transmittance of preferably 85% ormore, and more preferably 87% or more. A transmittance of 85% or moresufficiently ensures visibility. Although a higher total lighttransmittance of the polyester film is better, the total lighttransmittance is preferably 99% or less, or may be 97% or less, from thestandpoint of stable production.

The polyester film after heat treatment at 150° C. for 30 minutes has amaximum heat shrinkage of preferably 6% or less, and more preferably 5%or less. A heat shrinkage of 6% or less can reduce flatness errors, suchas curling or undulation, during HC processing. Although a lower heatshrinkage is better, the heat shrinkage is preferably −1% or more, andmore preferably 0% or more. A negative value means that the polyesterfilm expanded after heating. A value below −1% may also lead to flatnesserrors.

The polyester film for a foldable display according to the presentinvention can impart sufficient pencil hardness to a hard coating filmformed by laminating a hard coating layer on the polyester film. In thecase of conventional polyester films, a hard coating film formed bylaminating a hard coating layer on such a conventional polyester filmshows a decrease in pencil hardness seemingly due to deformation of thefilm in the thickness direction in pencil hardness evaluation of thehard coating film. The present invention can achieve a high level ofhardness in pencil hardness evaluation of hard coating films by settingthe indentation depth of the film in the thickness direction so as tofall within a specific range after unloading a test force with a dynamicultra-micro hardness tester, described later. The indentation depth ofthe film in the thickness direction after unloading a test force ispreferably 1.5 μm or less, more preferably 1.4 μm or less, and stillmore preferably 1.3 μm or less. When the indentation depth is 1.5 μm orless after unloading a test force (the final amount of deformation underload), the hard coating film formed by laminating a hard coating layerbecomes resistant to deformation in the thickness direction and shows ahigh level of pencil hardness in pencil hardness evaluation. A hardcoating film with a high level of pencil hardness makes it unlikely forthe display surface to have scratches and dents, increasing the displayvisibility. A lower indentation depth is better after unloading a testforce; however, from the standpoint of stable production and saturationof the effects, the indentation depth is preferably 0.3 μm or more, andmore preferably 0.5 μm or more.

In order to decrease the indentation depth after unloading a test force,it is effective to adjust the refractive index in the thicknessdirection to 1.52 or less. The means to adjust the refractive index to1.52 or less includes, for example, the following condition settingswithin the range in which other physical properties and the refractiveindex in the bending direction and in the folding direction can beregulated so as to fall within preferable ranges: adjusting the drawratio in the bending direction or in the folding direction to a highvalue, setting a low draw temperature in the bending direction or in thefolding direction, and setting a high heat fixation temperature. Theseare described later.

The surface of the polyester film in the present invention can besubjected to treatment for improving adhesion with a resin for forming,for example, a hard coating layer.

Examples of surface treatment methods include unevenness-formingtreatment by sandblasting, solvent treatment, etc.; and oxidationtreatment such as corona discharge, electron beam irradiation, plasmatreatment, ozone-UV irradiation, flame treatment, chromic-acidtreatment, and hot-air treatment. These methods can be used without anyrestriction.

Adhesion can also be improved by an adhesion-improving layer, such as aneasy-to-adhere layer. For the easy-to-adhere layer, resins such asacrylic resins, polyester resins, polyurethane resins, and polyetherresins can be used without any restriction. The easy-to-adhere layer canbe formed by a typical coating technique, preferably “an in-line coatingtechnique.”

The polyester film described above can be produced, for example, byperforming a polymerization step of homogenously dispersing inorganicparticles in a monomer solution, which is part of a polyester material,filtering the dispersion, and adding the filtrate to the remainder ofthe polyester material to polymerize a polyester; and a film-formingstep of melting and extruding the polyester into a sheet form through afilter, and cooling and drawing the sheet to form a substrate film.

Below, the method for producing a biaxially oriented polyester film isdescribed with an example in which pellets of polyethylene terephthalate(“PET” below) are used as a material of a substrate film. However, themethod is not limited to this example. Additionally, the example is notintended to limit the number of layers such as a monolayer or amultilayer.

After a predetermined proportion of PET pellets is mixed and dried, themixture is fed into a known extruder for melting and laminating, andthen extruded from the slit die into a sheet form, followed by coolingand solidifying the sheet on a casting roll to form an unstretched film.A monolayer can be produced with a single extruder. A multilayered film(i.e., including two or more layers) can be produced by laminatingmultiple film layers that each constitute an outermost layer using twoor more extruders, a multilayered manifold, or a confluence block (e.g.,confluence blocks with a square joint), extruding a sheet of two or morelayers from the outlet, and cooling the sheet on a casting roll toprepare an unstretched film.

In this case, it is preferable to perform high-precision filtration toremove foreign matter that may be present in the resin at any portion ofthe extruder at which the molten resin is maintained at about 280° C.during melt-extrusion. The filter material for use in high-precisionfiltration of a molten resin can be any material; however, a filtermaterial made of sintered stainless steel is preferable because it isexcellent in removing aggregates composed mainly of Si, Ti, Sb, Ge, orCu and organic matter with a high melting point.

Additionally, the filter material has a filtered particle size (initialfiltration efficiency: 95%) of preferably 20 μm or less, particularlypreferably 15 μm or less. A filtered particle size (initial filtrationefficiency: 95%) exceeding 20 μm may lead to insufficient removal offoreign matter with a size of 20 μm or more. Although high-precisionfiltration of molten resin using a filter material with a filteredparticle size of 20 μm or less (initial filtration efficiency: 95%) mayreduce productivity, such a filter material is preferable from thestandpoint of obtaining a film that has fewer protrusions caused bycoarse particles.

Refractive Index in Bending Direction

In the present invention, the refractive index of the polyester film atleast either in the longitudinal direction (machine direction) or in thewidth direction is preferably 1.590 to 1.620, and more preferably 1.591to 1.600. The refractive index of the polyester film in the bendingdirection is preferably 1.590 to 1.620, and more preferably 1.591 to1.600. The term “bending direction” as used here refers to a directionthat is orthogonal to the folding portion (reference numeral 21) assumedin the use of a foldable display, as indicated by reference numeral 22on a polyester film (reference numeral 2) shown in FIG. 2. A refractiveindex of 1.590 to 1.620 at least either in the longitudinal direction orin the width direction is preferable because such a refractive indexminimizes the deformation caused by repeatedly folding the foldabledisplay, and eliminates the risk of degrading image quality of thefoldable display. The refractive index is more preferably 1.591 to1.600. Of course, the direction of the refractive index is preferablythe bending direction. A refractive index of 1.590 or more enables thedisplay to maintain excellent visibility because it does not causecracks in the direction of the folding portion after a bending test,described later, and of course, does not cause fracture. The refractiveindex of a polyester film can be effectively adjusted by adjusting thedraw ratio and the draw temperature. Additionally, in order to adjustthe refractive index, relaxing in the drawing direction or multi-stagedrawing can be performed. In performing multi-stage drawing, it ispreferable to set the draw ratio in the second and subsequent stages toa higher ratio than the draw ratio in the first stage.

Fatigue due to compressive stress applied to the inside of the displaywhen the display is folded can be reduced by controlling the refractiveindex of the polyester film at least either in the longitudinaldirection (machine direction) or in the width direction, more preferablythe refractive index of the polyester film in the bending direction, soas to fall within the ranges above. Fatigue due to compressive stress isthought to occur mainly in the crystalline portions; fewer crystals inthe bending direction causes less fatigue. Thus, lowering the refractiveindex is thought to reduce the amount of crystals oriented in thebending direction and reduce compressive fatigue.

Additionally, the creep phenomenon caused by tensile stress applied tothe outer side of the folded display can be reduced by decreasing therefractive index. Fatigue due to tensile stress is thought to occurmainly in the amorphous portions; repeatedly applied stress causes themolecular chains to align, thus deforming the film. It is inferred thathaving fewer molecular chains aligned in the bending direction leads toless deformation caused by such an alignment of molecular chains.Because fatigue due to tension can be reduced by decreasing amorphousportions, higher crystallinity (i.e., density) is preferable.

In the present invention, the draw ratio of an unstretched polyestersheet either in the longitudinal direction (machine direction) or in thewidth direction is preferably 1.2-fold to 2.0-fold, and more preferably1.7-fold to 2.0-fold. The draw direction is preferably the bendingdirection. A draw ratio of 1.2-fold or more is preferable due to therebeing no deformation during post-processing such as hard-coatingapplication, while a draw ratio of 2.0-fold or less is preferablebecause the film thickness does not become uneven. The draw temperatureis preferably 75 to 120° C., and more preferably 75 to 105° C. Theheating method for use during drawing can be a known technique, such asa hot-air heating method, a roll heating method, or an infrared heatingmethod. A draw temperature of 75 to 120° C. can prevent the film fromhaving great unevenness in the thickness caused by drawing at a drawratio within the range described above. Additionally, the refractiveindex in the thickness direction can be reduced by drawing the film attemperatures as low as possible within the above range in which the filmdoes not have great unevenness in the thickness.

Refractive Index in the Direction of the Folding Portion

The refractive index of the polyester film in the direction orthogonalto the direction in which the refractive index is 1.590 to 1.620 ispreferably 1.670 to 1.700. Specifically, the refractive index in thedirection orthogonal to the bending direction (the direction of thefolding portion) is preferably 1.670 to 1.700. A refractive index of1.670 to 1.700 can reduce deformation that occurs when the film isfolded in the bending direction. A refractive index of 1.700 or less cansuppress the formation of cracks or fracture in the direction of thefolding portion. A refractive index of 1.670 or more can increaseflexibility in the bending direction and increase the surface hardness.A refractive index of 1.680 to 1.695 is more preferable. The refractiveindex in the direction orthogonal to the bending direction can beadjusted by, for example, the draw ratio, drawing preheatingtemperature, draw temperature, multi-stage drawing, and film relaxation.The draw ratio is preferably 4.0 to 6.0-fold, more preferably, 4.4 to6.0-fold. The drawing preheating temperature in the direction orthogonalto the bending direction is preferably 70 to 110° C. In performingmulti-stage drawing in the direction orthogonal to the bendingdirection, it is preferable to set the draw ratio in the second andsubsequent states higher than the draw ratio in the first stage. Filmrelaxation may be performed by 1 to 10% either in the machine direction(longitudinal direction) or in the vertical direction (width direction),or both.

Refractive Index in the Thickness Direction

The refractive index in the thickness direction is preferably 1.520 orless. This is because a refractive index in the thickness direction of1.520 or less can suppress the reduction in hardness of the film surfaceeven when the refractive index in the bending direction is designed tobe low, and can thus achieve both flexibility and surface hardness. Arefractive index in the thickness direction of 1.520 or less can reducethe indentation depth in the thickness direction after unloading a testforce and can increase the hardness of film surface, in particular thepencil hardness of a hard coating film formed by laminating a hardcoating layer. The refractive index in the thickness direction is morepreferably 1.515 or less, still more preferably 1.510 or less,particularly preferably 1.505 or less, and most preferably 1.500 orless. A lower refractive index in the thickness direction is preferable;however, from the standpoint of stable production, the refractive indexin the thickness direction is preferably 1.3 or more, and may be even1.4 or more. The refractive index in the thickness direction isparticularly preferably 1.410 or more. The refractive index in thethickness direction within these ranges can be achieved by increasingthe draw ratio in both the bending direction and the folding direction.In order to control the refractive index in the thickness directionafter controlling the refractive index in the bending direction and inthe width direction so as to fall within their preferable ranges, it ispreferable to set the conditions while checking the balance of eachprocess condition in the film forming process.

The refractive index in the thickness direction can be controlled so asto fall within the above ranges by drawing preheating temperature, drawtemperature, or draw ratio in the bending direction; or drawingpreheating temperature, draw temperature, multi-stage drawing,high-ratio drawing, or temperature setting for heat fixation in thedirection of the folding portion. The drawing preheating temperature inthe bending direction is preferably 70° C. to 110° C. The drawtemperature in the bending direction is preferably 75 to 120° C. Thedraw ratio in the bending direction is preferably 1.2-fold to 2.0-fold,and more preferably 1.7-fold to 2.0-fold. The refractive index in thethickness direction can be effectively reduced, while the flexibility inthe bending direction is maintained, by performing drawing with a lowdraw ratio at a low draw temperature. The drawing preheating temperaturein the direction of the folding portion is also preferably 75° C. to110° C. The draw temperature is preferably 75 to 120° C. The draw ratioin the folding portion is preferably 4.0-fold to 6.0-fold, and morepreferably 4.4-fold to 6.0-fold. The refractive index in the thicknessdirection can be effectively reduced while the refractive index in thebending direction is maintained or reduced. High-ratio drawing may beperformed by multi-stage drawing. In this case, setting the draw ratioin the second stage higher than the draw ratio in the first stage ispreferable because this enables effective control of the refractiveindex. The method of performing drawing again after a crystallizationstep can also be used. Accelerated drawing, in which the drawing rate isincreased from the beginning toward the latter half of the drawingprocess, may be used.

The heat fixation temperature is preferably 180 to 240° C. Heat fixationfacilitates crystallization oriented in the drawing direction andreduces the refractive index in the thickness direction. Although it isnot necessarily clear why the hardness of film surface is increased byreducing the refractive index in the thickness direction, it is presumedthat aromatic moieties such as benzene rings in the molecular chains areoriented in the plane direction, and this has the effect of suppressingdeformation caused by stress applied in the thickness direction.

Density of Polyester Film

The density of the polyester film is preferably 1.380 g/cm³ or more, andmore preferably 1.383 g/cm³ or more. A density of 1.380 g/cm³ or morecan increase flexibility and can increase the hardness of film surface,in particular the pencil hardness of a hard coating film formed bylaminating a hard coating layer on the polyester film. A higher densityis better. Although it somewhat depends on, for example, whetherparticles are present in the film, the density of the polyester film ispreferably 1.40 g/cm³or less. Setting the heat fixation temperatureduring film formation to 180 to 240° C. facilitates crystallization andthus effectively increases the density.

The bending direction of the polyester film is preferably thelongitudinal direction (machine direction). This makes it easier todecrease the refractive index in the bending direction in the secondstretching in biaxial stretching and increase flexibility. Morespecifically, a preferable polyester film can be obtained by drawing anunstretched polyester sheet in the longitudinal direction in a drawratio of 1.2 to 2.0-fold, more preferably. 1.7 to 2.0-fold.Additionally, in a preferable embodiment, the film is also drawn in thewidth direction in a draw ratio of 4.0 to 6.0-fold, more preferably 4.4to 6.0-fold.

In a particularly preferable embodiment of the present invention, thepolyester film satisfies the following four characteristicssimultaneously:

-   (1) the polyester film has a refractive index in the bending    direction of 1.590 to 1.620,-   (2) the polyester film has a refractive index in the direction of a    folding portion of 1.670 to 1.700,-   (3) the polyester film has a refractive index in the thickness    direction of 1.520 or less, and-   (4) the polyester film has a density of 1.380 g/cm³ or more.

However, the resulting film may not satisfy the four characteristicssimultaneously in the case of a combination of conditions that are notthe best options within the individual preferable production conditions(e.g., a combination of a draw ratio in the bending direction of1.4-fold or less, a draw ratio in the direction of the folding portionof less than 4.4-fold, and a heat fixation temperature of 220° C. orless), among the combinations within the scope of the preferableproduction conditions. In such a case, the four characteristics may beachieved simultaneously by fine-tuning some conditions or a combinationof them, such as increasing the draw ratio in the bending direction to1.7-fold or more, increasing the draw ratio in the direction of thefolding portion to 4.4-fold or more, increasing the heat fixationtemperature to about 230° C., and/or decreasing the draw temperature inthe bending direction and/or the direction of the folding portion.

To adjust film formability, film strength, thermal dimensionalstability, and poor appearance, any film forming method such as drawing,relaxation, heat fixation, and surface treatment may be used. In aparticularly preferable embodiment of the present invention, therefractive index and density of the film are controlled so as to fallwithin the preferable ranges above. Controlling the refractive index anddensity so as to fall within the preferable ranges above provides apolyester film that is suitable for foldable displays and that exhibitsexcellent flex resistance and surface hardness, in particular a highlevel of pencil hardness of a hard coating film formed by laminating ahard coating layer on the polyester film, as compared with conventionalfilms.

Specifically, for example, after PET pellets are sufficiently dried in avacuum, they are fed into an extruder, and then melted and extruded in asheet form at about 280° C., followed by cooling and solidifying thesheet to form an unstretched PET sheet. The obtained unstretched sheetis drawn 1.2-fold to 2.0-fold, more preferably 1.7-fold to 2.0-fold inthe longitudinal direction, with rolls heated to 75 to 120° C. to obtaina uniaxially oriented PET film. The film is then held with clips at itsends, and guided into a hot air zone heated to 75 to 120° C., and dried,followed by drawing 4.0-fold to 6.0-fold, more preferably 4.4-fold to6.0-fold in the width direction. Subsequently, the film is guided into aheat treatment zone at 180 to 240° C. and treated with heat for 1 to 60seconds. During the heat treatment step, the film may optionally besubjected to relaxation treatment by 0 to 10% in the width orlongitudinal direction.

The polyester film has an intrinsic viscosity of preferably 0.50 to 1.0dl/g. An intrinsic viscosity of 0.50 dl/g or more is preferable becausesuch an intrinsic viscosity increases the shock resistance of the film,and thus makes it unlikely for the internal circuitry of a display tobecome disconnected by an external shock. An intrinsic viscosity of 1.00dl/g or less is preferable because it prevents filtration pressure ofthe molten fluid from becoming too high, thus stabilizing filmproduction.

Typically, an organic EL display is used in foldable displays, and acircularly polarizing plate is provided on the viewing side in order toreduce reflected light. From the circularly polarizing plate, linearlypolarized light is emitted. Thus, when the display is observed throughpolarized sunglasses, the screen appears dark from some angles. Thisstate is generally called “blackout.” To prevent blackout, thecircularly polarizing plate is preferably provided such that the anglemade by the absorption axis of the circularly polarizing plate with theslow axis of the polymer film is 10 to 80°. Additionally, to minimizethe changes in brightness that occur when the display is rotated, theangle made by the absorption axis of the circularly polarizing platewith the slow axis of the polymer film is preferably 20 to 70°, morepreferably 30 to 60°, and particularly preferably 40 to 50°. Ideally,the angle is 45°.

When a display is observed through polarized sunglasses, coloring oriridescent color unevenness (rainbow unevenness) may appear due toin-plane retardation (Re) of the polymer film even if blackout does notoccur. This occurs for the following reason. When linearly polarizedlight emitted from the circularly polarizing plate passes through thepolymer film, the light is converted into linearly polarized light thatis orthogonal to the original polarized light or elliptically polarizedlight due to in-plane retardation (Re), which is the product of thebirefringence and the thickness of the polymer film. When the convertedpolarized light is observed through polarized sunglasses, a differencein transmittance arises depending on the state of polarization. Coloringand rainbow unevenness can be prevented by setting the in-planeretardation (Re) of the polymer film to 3000 nm or more. This is becauseof the following reason. The pattern of conversion of polarizationcomponents into linearly polarized light or elliptically polarized lightdepends on the wavelength and in-plane retardation (Re). This patterncan be repeated in a short wavelength cycle by setting a high in-planeretardation (Re). As a result, the shape of the envelope of thetransmitted light spectrum can approximate the emission spectrum of thelight source, and it becomes indistinguishable to the human eyes (seeFIG. 3).

The lower limit of in-plane retardation (Re) is preferably 4500 nm, andmore preferably 6000 nm.

The upper limit of in-plane retardation (Re) is preferably 30000 nm. Apolymer film having an in-plane retardation (Re) of more than 30000 nmwould not substantially provide an effect of further improvingvisibility, and may also increase the film thickness, thus decreasingflex resistance. The upper limit of in-plane retardation (Re) is morepreferably 20000 nm, still more preferably 15000 nm, particularlypreferably 12000 nm, and most preferably 10000 nm.

The in-plane retardation (Re) in the present invention can be determinedby measuring the refractive index in the biaxial direction and thethickness, or by using a commercially available automatic birefringenceanalyzer, such as a KOBRA-21ADH (Oji Scientific Instruments). The slowaxis direction can be determined by using a molecular orientationanalyzer (e.g., MOA-6004, molecular orientation analyzer, produced byOji Scientific Instruments).

The OLED light source used in foldable displays has a wide emissionspectrum. Thus, the shape of the envelope of the transmitted lightspectrum can approximate the emission spectrum of the light source withonly a simple structure by setting the in-plane retardation (Re) of thepolymer film to within the above ranges.

Easy-to-Adhere Layer

In the present invention, it is also preferable to laminate aneasy-to-adhere layer on the polyester film in order to improve theadhesiveness between the polyester film and the hard coating layer orother layers. The easy-to-adhere layer can be obtained by applying acoating solution for forming an easy-to-adhere layer to one or bothsurfaces of an unstretched or longitudinal, uniaxially oriented film,optionally performing heat treatment to dry the applied coating, anddrawing the film in at least one direction in which the film is notstretched. Heat treatment can also be performed after biaxial drawing isperformed. It is preferable to control the final amount of the appliedeasy-to-adhere layer to 0.005 to 0.20 g/m². An amount of the appliedeasy-to-adhere layer of 0.005 g/m² or more is preferable because itprovides adhesiveness. An amount of the applied easy-to-adhere layer of0.20 g/m²or less is preferable because it provides blocking resistance.

Examples of resins to be contained in the coating solution for use inlaminating an easy-to-adhere layer include polyester-based resins,polyether-polyurethane-based resins, polyester polyurethane resins,polycarbonate polyurethane resins, and acrylic resin; these resins canbe used without any particular limitation. Examples of crosslinkingagents to be contained in the coating solution for forming aneasy-to-adhere-layer include melamine compounds, isocyanate compounds,oxazoline compounds, epoxy compounds, and carbodiimide compounds. Thesecrosslinking agents can also be used in a combination of two or more.Due to the nature of in-line coating, these are preferably applied inthe form of an aqueous coating solution, and the resins and thecrosslinking agents are preferably water-soluble or water-dispersibleresins or compounds.

To impart smoothness to the easy-to-adhere layer, it is preferable toadd particles. The fine particles preferably have a mean particle sizeof 2 μm or less. Particles having a mean particle size of more than 2 μmare more likely to come off from the easy-to-adhere layer. Examples ofparticles to be contained in the easy-to-adhere layer include inorganicparticles, such as titanium oxide, barium sulfate, calcium carbonate,calcium sulfate, silica, alumina, talc, kaolin, clay, calcium phosphate,mica, hectorite, zirconia, tungsten oxide, lithium fluoride, and calciumfluoride, and organic polymer particles, such as styrene-basedparticles, acrylic-based particles, melamine-based particles,benzoguanamine-based particles, and silicone-based particles. These maybe singly added to the easy-to-adhere layer, or added in a combinationof two or more types.

The method for applying the coating solution for use may be a knownmethod as in the coating layer described above. Examples of methods forapplying the coating solution include reverse roll coating, gravurecoating, kiss coating, roll brush, spray coating, air-knife coating,wire-bar coating, and pipe doctor. These methods can be used singly orin combination.

Hard Coating Layer

When the polyester film in the present invention is used as a surfaceprotection film for a foldable display by positioning the polyester filmon the surface of the display, the polyester film preferably has a hardcoating layer on at least one surface of the film. The hard coatinglayer is preferably used in a display by positioning on the surface ofthe polyester film that is the front surface of the display. The resinfor forming a hard coating layer can be any resin without particularlimitations, such as acrylic resins, siloxane resins, inorganic hybridresins, urethane acrylate resins, polyester acrylate resins, and epoxyresins. These resins may also be used in a combination of two or more.The resin for use may contain particles such as inorganic filler ororganic filler.

Film Thickness of Hard Coating Layer

The film thickness of the hard coating layer is preferably 1 to 50 μm. Afilm thickness of 1 μm or more is preferable because it ensuressufficient curing and leads to a high level of pencil hardness. A filmthickness of 50 μm or less suppresses curling caused by hardening andcontraction of the hard coating, thus increasing film handleability.

Coating Method

The method for forming a hard coating layer for use may be, for example,a Mayer bar, gravure coater, die coater, or knife coater. These methodscan be used without any particular limitation and can be suitablyselected according to viscosity and film thickness.

Curing Conditions

The method for curing the hard coating layer for use may be a methodusing energy beams such as UV light or electron beams, or a method usingheat, From the standpoint of minimizing damage to the film, a methodusing energy beams such as UV light or electron beams is preferable.

Pencil Hardness

The pencil hardness of the hard coating layer is preferably 3H orhigher, and more preferably 4H or higher. A pencil hardness of 3H orhigher prevents the hard coating layer from being easily scratched,while not decreasing visibility. Although a higher level of pencilhardness of the hard coating layer is generally better, the pencilhardness may be 9H or lower or 8H or lower; even a hard coating layerwith a pencil hardness of 6H or lower is usable in practice withoutproblems.

Properties of Hard Coating Layer

The hard coating layer in the present invention can be used for thepurpose of protecting a display by increasing the pencil hardness of thesurface as described above, and preferably has high transmittance. Thehard coating film has a transmittance of preferably 87% or more, andmore preferably 88% or more. A transmittance of 87% or more ensuressufficient visibility. In general, a higher total light transmittance ofthe hard coating film is preferable. However, from the standpoint ofstable production, the total light transmittance of the hard coatingfilm is preferably 99% or less, and may be 97% or less. In general, thehard coating film preferably has a lower haze, and preferably a haze of3% or less. The hard coating film has a haze of more preferably 2% orless, and most preferably 1% or less. A haze of 3% or less can improvethe visibility of images. Although a lower haze is generally better, thehaze of the hard coating film is preferably 0.1% or more, or may be 0.3%or more, from the standpoint of stable production.

The hard coating layer may have further functions added. For example, ahard coating layer with additional functionality, such as an anti-glarelayer, anti-glare anti-reflective layer, anti-reflective layer,low-reflection layer, or antistatic layer having the predeterminedpencil hardness above, can also be preferably used in the presentinvention.

The polyester film used as a substrate film of a touchscreen module mayalso be provided with a hard coating layer. For example, when an ITOlayer is used as a transparent electrode layer of a touchscreen module,a refractive index adjustment layer is preferably provided between thesubstrate film and the transparent electrode layer to make the electrodepattern less visible. In this case, the hard coating layer itself mayserve as a refractive index adjustment layer, or a separate refractiveindex adjustment layer may be laminated.

EXAMPLES

The following describes the present invention with reference to Examplesand Comparative Examples. First, the methods for evaluatingcharacteristic values used in the present invention are described below.

(1) Intrinsic Viscosity

A film or polyester resin was crushed, dried, and dissolved in a mixedsolvent of phenol and tetrachloroethane in a ratio of phenol totetrachloroethane of 60/40 (mass ratio). This solution was thencentrifuged to remove inorganic particles. The flow time of the solutionwith a concentration of 0.4 (g/dl) and the flow time of the solventalone were measured with an Ubbelohde viscometer at 30° C. From the timeratio, the intrinsic viscosity was calculated by using the Hugginsequation with the assumption of Huggins's constant being 0.38.

(2) Flex Resistance of Polyester Film Sample (Bend Radius: 1.5 mm)

A polyester film sample with a size of 20 mm in the width direction×110mm in the machine direction was prepared. The sample was folded 200,000times at a rate of 1 time/second with a DLDMLH-FS tension-free U-shapefolding tester (produced by Yuasa System Co., Ltd.) with the bend radiusset to 1.5 mm. During the process, the sample was fixed at 10 mm fromboth ends of the long side, and the area to be folded was 20 mm×90 mm.FIG. 1 is a schematic diagram showing the bend radius of the foldabledisplay being folded. A bending test was performed as a model test withthe portion indicated by numerical reference 11 in FIG. 1 set to 1.5 mm,assuming the case in which a polyester film was provided on the innersurface of the folded display. After folding was ended, the sample wasplaced on a flat surface with the inner surface of the folded samplefacing down and visually observed.

-   A: No cracks or deformation can be confirmed on the sample.-   B: The sample has a crack or a crease, and the maximum height of the    lifted portion is 5 mm or more when it is placed horizontally.

(3) Flex Resistance of Polyester Film Sample (Bend Radius: 0.5 mm)

In the same manner as in the above bending test, a sample was folded200,000 times at a rate of 1 time/second with the bend radius set to 0.5mm. FIG. 1 is a schematic diagram showing the bend radius of a foldabledisplay being folded. A bending test was performed as a model test withthe portion indicated by numerical reference 11 in FIG. 1 set to 0.5 mm,assuming the case in which a polyester film was provided on the innersurface of the folded display. The film surface on the outer side of thefolded portion was observed with a digital microscope (RH8800, producedby HIROX) at 700×to examine whether creases (cracks) were present.Separately from the above visual test of flex resistance with a bendingradius of 1.5 mm, this test, performed with a smaller bending radius of0.5 mm, was intended to evaluate a foldable display to which the hardcoating layer and other materials are laminated or adhered underconditions close to the actual use conditions of the foldable display.Separately from the visual observation with the bending radius set to1.5 mm, this test was performed to detect defects that are thelikelihood of causing breaks or cracks, which are minute defectsdifficult to detect visually.

-   A: No defect is observed on the film surface of the outer side the    folded portion.-   B: The display has been broken, or creases (cracks) can be observed    on the film surface of the outer side of the folded portion.

(4) Refractive Index

In accordance with JIS K 7142:2008 (Plastic: Determination of refractiveindex (method A)), the refractive index in the longitudinal direction,the refractive index in the width direction, and the refractive index inthe thickness direction were determined with an Abbe refractometer(NAR-4T, produced by Atago Co., Ltd., measurement wavelength: 589 nm).

(5) Pencil Hardness

Pencil hardness was measured under a load of 750 g at rate of 1.0 mm/swith the pencil hardness of a hard coating film as a sample inaccordance with JIS K 5600-5-4:1999. In the present invention, a pencilhardness of 3H or more was rated as passing.

(6) Total Light Transmittance and Haze

Total light transmittance and haze were measured with a NDH5000 hazemeter (produced by Nippon Denshoku Industries Co., Ltd.).

(7) Density

Density was measured in accordance with the method described in JIS K7112:1999 (density-gradient tube method) (unit: g/cm.³).

(8) Indentation Depth after Test Force is Unloaded

A sample was cut to a size of about 2 cm×2 cm, and the surface oppositethe surface to be measured was immobilized on a micro cover glass (18×18mm, produced by Matsunami Glass Ind., Ltd.) with an adhesive (Cemedine®High-super 30). After being adhered and immobilized, the sample wasallowed to stand for at least 12 hours at room temperature. Thereafter,the sample was measured for indentation depth (μm) after a test forcewas unloaded with a DUH-211 dynamic ultra-micro hardness tester(Shimadzu Corporation) under the following conditions.

Measurement Condition

-   Test mode: loading-unloading test-   Indenter for use: edge angle 115°, triangular pyramid indenter-   Indenter elasticity: 1.140×10⁶N/mm²-   Indenter Poisson's ratio: 0.07-   Test force: 50 mN-   Loading rate: 4.44 mN/sec-   Loading retention time: 2 sec-   Unloading retention time: 0 sec

(9) Maximum Heat Shrinkage

A sample film was cut to a size of 10 mm (length)×250 mm (width), and along side was aligned with the direction to be measured and marked atintervals of 200 mm. Distance A, which is between the marks, wasmeasured under constant tension of 5 g. Subsequently, the sample filmwas allowed to stand in atmosphere at 150° C. in an oven for 30 minuteswithout a load, and then taken out of the oven, followed by cooling toroom temperature. Thereafter, distance B, which is between the marks,was measured under constant tension of 5 g, and the heat shrinkage (%)was determined by using the following formula. The heat shrinkage wasmeasured at three evenly separated points in the width direction of thesample film, and the average of the values at the three points was takenas the heat shrinkage (b).

Heat shrinkage (%)=[(A−B)×100]/A

The sample film was cut so that the vertical and horizontal directionswere different for both the bending direction and the folding direction,and measurement was performed. The data of the direction larger inmeasurement value was taken as the maximum heat shrinkage (%).

(10) In-Plane Retardation (Re)

In-plane retardation refers to a parameter defined as the product (ΔN×d)of anisotropy of the refractive indices of two axes orthogonal to eachother on the film (ΔN=|Nx−Ny|) and the film thickness d (nm), and is ameasure of optical isotropy or anisotropy. The anisotropy (ΔN) ofrefractive indices of two axes was determined by the following method.The axis orientation directions of a film were determined by using twopolarization plates. The film was cut to a rectangle of 4 cm×2 cm suchthat the axis orientation directions were orthogonal to each other, andthe cut piece was taken as a measurement sample. For this sample, therefractive indices (Nx,Ny) of the two axes that are orthogonal to eachother and the refractive index in the thickness direction (Nz) weredetermined with an Abbe refractometer (NAR-4T, produced by Atago Co.,Ltd.). The absolute value of the difference between the refractiveindices of the two axes (|Nx−Ny|) was taken as the anisotropy (ΔN) ofthe refractive indices. The thickness d (nm) of the film was measuredwith an electric micrometer (Millitron 1245D, produced by FeinprufGmbH), and the value was converted to the unit nm. From the product(ΔN×d) of the anisotropy (ΔN) of refractive indices and the filmthickness d (nm), in-plane retardation (Re) was determined. Of theoriented axes, the axis showing a larger refractive index is defined asa slow axis.

(11) Rainbow Unevenness Observation

An after-window film on the surface of an OLED tablet (Samsung GalaxyTab S3 SM-T820, 9.7-inch, 32 GB tablet) was peeled off. Instead, thepolyester film of the present invention was adhered to the surface byusing a commercially available substrate-free optical adhesive sheetsuch that the bending direction of the polyester film was aligned withthe longitudinal direction of the tablet. The absorption axis of thecircularly polarizing plate of the tablet was 45° to the longitudinaldirection of the tablet, and 45° to the slow axis direction of thepolyester film.

The OLED tablet was set to white screen display (R=0, G=0, B=0) andobserved from the front and from an oblique direction through polarizedsunglasses to determine whether blackout and/or rainbow unevenness waspresent as described below. Color irregularities observed only on thetablet used were excluded from the determination. Blackout wasdetermined to be present when blackout appeared from the front of thetablet at some angle while the tablet was rotated within the plane.

-   A: No blackout is observed, and also no rainbow unevenness is    observed in the front and oblique directions.-   B: No blackout is observed, but some very light rainbow unevenness    is observed from an oblique direction.-   C: No blackout is observed, but rainbow unevenness is observed from    an oblique direction.-   D: Blackout is observed.

Preparation of Polyethylene Terephthalate Pellet

The esterification reactor for use was a continuous esterificationreactor composed of a three-stage complete mixing tank equipped with astirrer, a partial condenser, a feedstock inlet, and a product outlet.Slurry of TPA in an amount of 2 tons/hr, 2 mol of EG per mol of TPA, andantimony trioxide in an amount of 160 ppm on an Sb atom basis relativeto a produced PET was continuously supplied to the first esterificationreaction vessel of the esterification reactor, and allowed to react at255° C. under ordinary pressure with an average residence time of 4hours. Subsequently, the reaction product in the first esterificationreaction vessel was continuously taken out of the system and supplied tothe second esterification reaction vessel. EG distilled from the firstesterification reaction vessel in an amount of 8 mass % of the producedpolymer (produced PET) was then supplied to the second esterificationreaction vessel, and an EG solution containing magnesium acetate in anamount of 65 ppm on a Mg atom basis relative to the produced PET, and anEG solution containing TMPA in an amount of 20 ppm on a P atom basisrelative to the produced PET were further added thereto, followed by areaction at 260° C. under ordinary pressure with an average residencetime of 1.5 hours. Subsequently, the reaction product in the secondesterification reaction vessel was continuously taken out of the systemand supplied to the third esterification reaction vessel, and an EGsolution containing TMPA in an amount of 20 ppm on a P atom basisrelative to the produced PET was further added thereto, followed by areaction at 260° C. under ordinary pressure with an average residencetime of 0.5 hours. The esterified reaction product generated in thethird esterification reaction vessel was continuously supplied to athree-stage continuous polycondensation reactor to performpolycondensation, and filtered through a sintered stainless-steel filtermaterial (nominal filtration accuracy: 5-μm particles are 90% cut),thereby obtaining polyethylene terephthalate pellet (a) with anintrinsic viscosity of 0.62 dl/g.

Preparation of Polyethylene Terephthalate Pellet (b)

Polyethylene terephthalate pellet (b) was obtained in the same manner asabove by adjusting the intrinsic viscosity to 0.580 dl/g, except thatthe residence time of the third esterification reaction was adjusted inthe production step of polyethylene terephthalate pellet (a).

Preparation of Polyethylene Terephthalate Pellet (c)

Polyethylene terephthalate pellet (c) with an intrinsic viscosity of0.75 dl/g was obtained by subjecting polyethylene terephthalate pellet(a) to solid-state polymerization at 220° C. under a reduced pressure of0.5 mmHg for a different period of time with a rotary vacuumpolymerizer.

Polymerization of Urethane Resin

72.96 parts by mass of 1,3-bis(methylisocyanate) cyclohexane, 12.60parts by mass of dimethylol propionic acid, 11.74 parts by mass ofneopentyl glycol, 112.70 parts by mass of polycarbonate diol with anumber average molecular weight of 2000, and as solvents, 85.00 parts bymass of acetonitrile and 5.00 parts by mass of N-methylpyrrolidone wereplaced in a four-necked flask equipped with a stirrer, a Dimrothcondenser, a nitrogen-feeding tube, a silica-gel-drying tube, and athermometer, and stirred in a nitrogen atmosphere at 75° C. for 3 hours;the reaction mixture was confirmed to have reached a predetermined amineequivalent. Subsequently, after this reaction mixture was cooled to 40°C., 9.03 parts by mass of triethyl amine was added, thereby obtaining apolyurethane prepolymer D solution. Subsequently, 450 g of water wasadded to a reactor equipped with a homogenizing disperser capable ofhigh-speed stirring, and the temperature was adjusted to 25° C.,followed by dispersing an isocyanate-terminated prepolymer in water withstirring at 2000 min-1. Thereafter, some acetonitrile and water wereremoved under reduced pressure, thereby preparing water-solublepolyurethane resin (A) with a solids content of 35 mass %.

Polymerization of Water-soluble Carbodiimide Compound

200 parts by mass of isophorone diisocyanate and 4 parts by mass of3-methyl-1-phenyl-2-phosphorene-1-oxide (carbodiimidized catalyst) wereadded to a flask equipped with a thermometer, a nitrogen-gas-feedingtube, a reflux condenser, a dropping funnel, and a stirrer, and stirredin a nitrogen atmosphere at 180° C. for 10 hours, thereby obtaining anisocyanate-terminated isophorone carbodiimide (degree of polymerization:5). Subsequently, 111.2 g of the obtained carbodiimide and 80 g ofpolyethylene glycol monomethyl ether (molecular weight: 400) werereacted at 100° C. for 24 hours. Water was gradually added thereto at50° C., thereby obtaining transparent yellowish water-solublecarbodiimide compound (B) with a solids content of 40 mass.

Preparation of Coating Solution for Forming Easy-to-Adhere-Layer

The following coating materials were mixed, thereby preparing a coatingsolution.

-   Water: 16.97 parts by mass-   Isopropanol: 21.96 parts by mass-   Polyurethane resin (A): 3.27 parts by mass-   Water-soluble carbodiimide compound (B): 1.22 parts by mass-   Particles: 0.51 parts by mass-   (silica sol with a mean particle size of 40 nm, solids    concentration: 40 mass)-   Surfactant: 0.05 parts by mass-   (silicone-based surfactant, solids concentration: 100 mass %)

Preparation of Hard Coating Solution a

0.1 parts by weight of a leveling agent (produced by BYK-Chemie Japan,BYK307, concentration: 100%) were added to 100 parts by weight of a hardcoating material (produced by JSR Corporation, OPSTAR® Z7503,concentration: 75%), and the mixture was diluted with methyl ethylketone, thereby preparing a hard coating solution a with a solidsconcentration of 40 wt %.

Example 1

Polyethylene terephthalate pellet (a) was supplied to an extruder andmelted at 285° C. This polymer was filtered through a sinteredstainless-steel filter material (nominal filtration accuracy: 10-μmparticles are 95% cut) and extruded from the outlet into a sheet form.The sheet-form polymer was then brought into contact with a casting drum(surface temperature: 30° C.) by using an electrostatic applicationcasting method to solidify the polymer by cooling, thereby preparing anunstretched film. The unstretched film was uniformly heated to 75° C. byusing heating rolls, and then heated to 85° C. using a non-contactheater, followed by roll drawing (drawing in the longitudinal direction)to a 1.4-fold film. Subsequently, the coating solution for forming aneasy-to-adhere layer was applied to the both surfaces of the obtaineduniaxially stretched film by roll coating, and then dried at 80° C. for20 seconds. Adjustment was made so that the amount of the appliedcoating solution for forming an easy-to-adhere layer was 0.06 g/m² afterfinal drying (after being biaxially drawn). Thereafter, the film wasguided to a tenter, preheated at 105° C., and laterally stretched4.0-fold at 95° C. With the width fixed, the film was subjected to heatfixation at 230° C. for 5 seconds, and further relaxed by 4% in thewidth direction at 180° C., thereby obtaining a polyethyleneterephthalate film with a thickness of 50 μm. Tables 1 and 2 show theevaluation results.

Examples 2 and 3

A polyester film was prepared in the same manner as in Example 1, exceptthat the draw ratio in the longitudinal direction was changed as shownin Table 1.

Example 4

A polyester film was prepared in the same manner as in Example 1, exceptthat the draw ratio in the width direction was changed to 4.4-fold, andthe heat fixation temperature was changed to 220° C.

Examples 5 and 6

A polyester film was prepared in the same manner as in Example 4, exceptthat the draw ratio in the longitudinal direction was changed as shownin Table 1.

Example 7

A polyester film was prepared in the same manner as in Example 1, exceptthat the draw ratio in the width direction was changed to 5.5-fold, andthe heat fixation temperature was changed to 190° C.

Examples 8 and 9

A polyester film was prepared in the same manner as in Example 7, exceptthat the draw ratio in the longitudinal direction was changed as shownin Table 1.

Example 10

A polyester film was prepared in the same manner as in Example 5, exceptthat in the production step of Example 5, the film was drawn in thelongitudinal direction, and then subjected to relaxation heat treatmentby 10% at 100° C.

Example 11

A polyester film was prepared in the same manner as in Example 5, exceptthat in the production step of Example 5, the film was released from theclips at 200° C. after heat fixation, and the film was subjected torelaxation heat treatment in the longitudinal direction and in the widthdirection. In the longitudinal direction, the tenter rate and thetake-up roll rate were adjusted to achieve a relaxation rate of 3%. Therelaxation in the width direction was set to be free.

Example 12

A polyester film was prepared in the same manner as in Example 1, exceptthat the temperature during drawing in the longitudinal direction waschanged to 75° C., and the heat fixation temperature was changed to 220°C.

Example 13

A polyester film was prepared in the same manner as in Example 1, exceptthat drawing in the longitudinal direction was performed by changing thetemperature to 75° C. and the draw ratio to 1.2-fold, and then drawingin the width direction was performed by changing the draw ratio to5.0-fold.

Example 14

A polyester film was prepared in the same manner as in Example 3, exceptthat the drawing in the longitudinal direction in Example 3 was changedto two-stage drawing, the draw ratio in the first stage was 1.2-fold,and the draw ratio in the second stage was 1.67-fold. The total drawratio in the longitudinal direction was about 2.0-fold.

Example 15

A polyester film was prepared in the same manner as in Example 5, exceptthat the preheating temperature during the drawing in the widthdirection was changed to 95° C., and the heat fixation temperature waschanged to 190° C.

Example 16

A polyester film was prepared in the same manner as in Example 2, exceptthat the drawing in the width direction in Example 2 was changed totwo-stage drawing, the draw ratio in the first stage was 1.5-fold, thedraw ratio in the second stage was 4.0-fold, and the heat fixationtemperature was changed to 190° C. The total draw ratio in the widthdirection was 6.0-fold.

Examples 17 and 18

A polyester film was prepared in the same manner as in Example 2, exceptthat the thickness was changed as shown in Table 2.

Example 19

A polyester film was prepared in the same manner as in Example 1, exceptthat in the production step of Example 1, relaxation heat treatment inthe width direction was not performed.

Example 20

In the same manner as in Example 1, an unstretched film was prepared,and then the unstretched film was preheated at 75° C. with a tenter,.followed by laterally drawing the film 1.4-fold at 85° C. The coatingsolution for forming an easy-to-adhere layer was applied to the bothsurfaces of the obtained uniaxially stretched film by roll coating anddried at 80° C. for 20 seconds. Adjustment was made so that the amountof the applied coating solution for forming an easy-to-adhere layer was0.06 g/m² after final drying (after being biaxially drawn). The film wasuniformly heated to 105° C. by using heating rolls, and then heated to95° C. using a non-contact heater, followed by roll drawing (drawing inthe longitudinal direction) to a 4.0-fold film. With the width fixed,the film was subjected to heat fixation at 230° C. for 5 seconds,thereby obtaining a polyethylene terephthalate film with a thickness of50 μm.

Comparative Example 1

A polyester film was prepared in the same manner as in Example 1, exceptthat drawing in the longitudinal direction was not performed, and onlylateral uniaxial drawing in the width direction was performed.

Comparative Example 2

A polyester film was prepared in the same manner as in Example 7, exceptthat drawing in the longitudinal direction was not performed, and onlylateral uniaxial drawing in the width direction was performed.

Comparative Examples 3 to 7

A polyester film was prepared in the same manner as in Example 1, exceptthat the heat fixation temperature was changed to 220° C., and the PETpellet shown in Table 1 was used, while the thickness was changed asshown in Table 2. In Comparative Examples 3 to 7, the heat fixationtemperature was lower than in Example 1, and the combination of the drawratio in the longitudinal direction and the draw ratio in the widthdirection was not the best, given the preferable range of each drawratio. Thus, as shown in Table 2, the refractive index in the thicknessdirection was increased, the indentation depth after unloading the testforce was large, and the pencil hardness after lamination of the hardcoating layer was also lower than in the Examples.

Comparative Example 8

A polyester film was prepared in the same manner as in Example 1, exceptthat the draw ratio in the longitudinal direction was changed to2.7-fold, and the heat fixation temperature was changed to 220° C.

Comparative Example 9

A polyester film was prepared in the same manner as in Example 1, exceptthat the draw ratio in the longitudinal direction was changed to3.4-fold.

Comparative Example 10

A polyester film was prepared in the same manner as in Example 4, exceptthat the heat fixation temperature was changed to 100° C.

Comparative Example 11

A polyester film was prepared in the same manner as in Example 13,except that the draw temperature in the longitudinal direction waschanged to 130° C.

Comparative Example 12

A polyester film was prepared in the same manner as in Example 1, exceptthat the preheating temperature in the width direction was changed to120° C.

Comparative Example 13

A polyester film was prepared in the same manner as in Example 13,except that the draw ratio in the width direction was changed to5.5-fold, and the heat fixation temperature was changed to 230° C.

Hard coating solution a was applied to one surface of each of theprepared films by using a Mayer bar such that the film thickness was 5pm on a dry film basis, and dried at 80° C. for 1 minute. The films werethen irradiated with UV light (integrated light intensity: 200 mJ/cm²),thereby obtaining hard coating films.

The hard coating films were each laminated on an organic EL module via a25-μm-thick adhesive layer, thereby preparing foldable smartphone typedisplays that can be folded in half at their center, with a radius of 3mm, which corresponds to the bend radius in FIG. 1. Each hard coatingfilm was provided on the surface of the single continuous display viathe folding portion so that the hard coating layer was positioned as thefront surface of the display. The displays prepared using the hardcoating films of the Examples were satisfactory in terms of operationand visibility as a portable smartphone that is foldable in half attheir center. The surface of these displays was also not dented byexternal forces. On the other hand, the foldable displays prepared byusing the hard coating films of the Comparative Examples were not sodesirable because they appeared to develop image distortion at thefolding portion of the display as the frequency of use increased.Additionally, some had dents and scratches on the surface.

TABLE 1 PET Film Drawing Temperature Preheating PET Pellet inTemperature Intrinsic Intrinsic Draw Ratio Longitudinal In Width HeatFixation Viscosity Viscosity Longitudinal Width Direction DirectionTemperature Relaxation Relaxation Type (dl/g) (dl/g) Direction Direction(° C.) (° C.) (° C.) Direction Rate (%) Example 1 (a) 0.62 0.58 1.4 4.085 105 230 Width 4 Direction Example 2 (a) 0.62 0.58 1.7 4.0 85 105 230Width 4 Direction Example 3 (a) 0.62 0.58 2.0 4.0 85 105 230 Width 4Direction Example 4 (a) 0.62 0.58 1.4 4.4 85 105 220 Width 4 DirectionExample 5 (a) 0.62 0.58 1.7 4.4 85 105 220 Width 4 Direction Example 6(a) 0.62 0.58 2.0 4.4 85 105 220 Width 4 Direction Example 7 (a) 0.620.58 1.4 5.5 85 105 190 Width 4 Direction Example 8 (a) 0.62 0.58 1.75.5 85 105 190 Width 4 Direction Example 9 (a) 0.62 0.58 2.0 5.5 85 105190 Width 4 Direction Example 10 (a) 0.62 0.58 1.7 4.4 85 105 220Longitudinal 10 Direction Example 11 (a) 0.62 0.58 1.7 4.4 85 105 220Longitudinal/ 3/− Width Example 12 (a) 0.62 0.58 1.4 4.0 75 105 220Width 4 Direction Example 13 (a) 0.62 0.58 1.2 5.0 75 105 220 Width 4Direction Example 14 (a) 0.62 0.58 2.0 4.0 85 105 230 Width 4 (2 stages)Direction Example 15 (a) 0.62 0.58 1.7 4.4 85 95 100 Width 4 DirectionExample 16 (a) 0.62 0.58 1.7 6.0 85 105 190 Width 4 (2 stages) DirectionExample 17 (a) 0.62 0.58 1.7 4.0 85 105 230 Width 4 Direction Example 18(a) 0.62 0.58 1.7 4.0 85 105 230 Width 4 Direction Example 19 (a) 0.620.58 1.4 4.0 85 105 230 — 0 Example 20 (a) 0.62 0.58 4.0 1.4 105 85 230— 0 Comparative (a) 0.62 0.58 1.0 4.0 — 105 220 Width 4 Example 1Direction Comparative (a) 0.62 0.58 1.0 5.5 — 105 190 Width 4 Example 2Direction Comparative (a) 0.62 0.58 1.4 4.0 85 105 220 Width 4 Example 3Direction Comparative (a) 0.42 0.58 1.4 4.0 85 105 220 Width 4 Example 4Direction Comparative (a) 0.62 0.58 1.4 4.0 85 105 220 Width 4 Example 5Direction Comparative (b) 0.58 0.54 1.4 4.0 85 105 220 Width 4 Example 6Direction Comparative (c) 0.75 0.69 1.4 4.0 85 105 220 Width 4 Example 7Direction Comparative (a) 0.62 0.58 2.7 4.0 85 105 220 Width 4 Example 8Direction Comparative (a) 0.62 0.58 3.4 4.0 85 105 230 Width 4 Example 9Direction Comparative (a) 0.62 0.58 1.4 4.4 85 105 100 Width 4 Example10 Direction Comparative (a) 0.62 0.58 1.2 5.0 130 105 220 Width 4Example 11 Direction Comparative (a) 0.62 0.58 1.4 4.0 85 120 230 Width4 Example 12 Direction Comparative (a) 0.62 0.58 1.2 5.5 85 105 230Width 4 Example 13 Direction

TABLE 2 PET Film Continuous Continuous Refractive Index Bending TestBending Test Thickness Density Longitudinal Width Thickness Bending withBend Radius with Bend Radius (μm) (g/cm³) Direction Direction DirectionDirection of 1.5 mm of 0.5 mm Example 1 50 1.385 1.506 1.684 1.516Longitudinal A A Direction Example 2 50 1.385 1.602 1.681 1.512Longitudinal A A Direction Example 3 50 1.387 1.609 1.879 1.509Longitudinal A A Direction Example 4 50 1.383 1.592 1.69 1.517Longitudinal A A Direction Example 5 50 1.383 1.597 1.688 1.515Longitudinal A A Direction Example 8 50 1.384 1.598 1.688 1.513Longitudinal A A Direction Example 7 50 1.383 1.591 1.694 1.513Longitudinal A A Direction Example 8 50 1.383 1.598 1.690 1.512Longitudinal A A Direction Example 9 50 1.383 1.597 1.688 1.513Longitudinal A A Direction Example 10 50 1.385 1.594 1.689 1.515Longitudinal A A Direction Example 11 50 1.385 1.596 1.687 1.515Longitudinal A A Direction Example 12 50 1.385 1.606 1.684 1.518Longitudinal A A Direction Example 13 50 1.386 1.591 1.685 1.519Longitudinal A A Direction Example 14 50 1.388 1.806 1.881 1.511Longitudinal A A Direction Example 15 50 1.383 1.598 1.691 1.495Longitudinal A A Direction Example 16 50 1.384 1.594 1.895 1.508Longitudinal A A Direction Example 17 25 1.387 1.602 1.681 1.512Longitudinal A A Direction Example 18 75 1.388 1.602 1.681 1.512Longitudinal A A Direction Example 19 50 1.384 1.598 1.887 1.513Longitudinal A A Direction Example 20 50 1.385 1.686 1.593 1.516 Width AA Direction Comparative 50 1.380 1.588 1.894 1.522 Longitudinal A BExample 1 Direction Comparative 50 1.383 1.584 1.701 1.512 LongitudinalA B Example 2 Direction Comparative 50 1.381 1.601 1.684 1.524Longitudinal A A Example 3 Direction Comparative 25 1.381 1.501 1.6761.530 Longitudinal A A Example 4 Direction Comparative 75 1.381 1.6231.590 1.528 Longitudinal B A Example 5 Direction Comparative 50 1.3821.568 1.682 1.524 Longitudinal A A Example 8 Direction Comparative 501.380 1.603 1.686 1.522 Longitudinal A A Example 7 Direction Comparative50 1.398 1.631 1.588 1.500 Longitudinal B A Example 8 DirectionComparative 50 1.306 1.650 1.869 1.496 Longitudinal B A Example 9Direction Comparative 50 1.384 1.578 1.660 1.532 Longitudinal B AExample 10 Direction Comparative 50 1.385 1.589 1.685 1.525 LongitudinalA B Example 11 Direction Comparative 50 1.384 1.596 1.879 1.526Longitudinal A A Example 12 Direction Comparative 50 1.383 1.502 1.7011.504 Longitudinal A B Example 13 Direction PET Film IndentationIn-Plane Maximum Hard Depth after Retardation Rainbow Total Light HeatCoating Film Unloading (Re) Unevenness Transmittance Haze ShrinkagePencil (μm) ΔN (nm) Observation (%) (%) (%) Hardness Example 1 1.480.088 4385 C to B 91 0.8 1.4 3 H Example 2 1.47 0.079 3937 C to B 91 0.81.6 3 H Example 3 1.42 0.070 3617 B 91 0.8 1.8 3 H Example 4 1.48 0.0984900 B 91 0.8 1.7 3 H Example 6 1.45 0.091 4550 B 91 0.8 1.9 3 H Example6 1.4 0.088 4400 C to B 91 0.8 2.2 3 H Example 7 1.36 0.103 5165 A 910.8 4.4 3 H Example 8 1.38 0.094 4705 B 91 0.8 4.9 3 H Example 9 1.410.091 4567 B 91 0.8 6.1 3 H Example 10 1.45 0.096 4760 B 91 0.8 1.0 3 HExample 11 1.45 0.091 4650 B 91 0.8 0.8 3 H Example 12 1.48 0.078 3915 Cto B 91 0.8 1.6 3 H Example 13 1.48 0.094 4700 B 91 0.8 1.6 3 H Example14 1.43 0.075 3750 C to B 91 0.8 1.6 3 H Example 15 1.27 0.093 4660 B 910.8 6.0 3 H Example 16 1.26 0.101 6060 B 91 0.8 4.7 3 H Example 17 1.470.079 1968 C 91 0.8 1.4 3 H Example 18 1.47 0.079 5905 A 91 0.8 1.6 3 HExample 19 1.48 0.089 4450 C to B 91 0.8 2.0 3 H Example 20 1.48 0.0934660 B 91 0.8 1.6 3 H Comparative 1.64 0.106 5260 B 91 0.8 1.4 1 HExample 1 Comparative 1.56 0.118 5886 A 91 0.8 3.7 2 H Example 2Comparative 1.60 0.083 4155 C to B 91 0.8 1.8 2 H Example 3 Comparative1.62 0.085 2120 C 91 0.8 1.8 2 H Example 4 Comparative 1.63 0.087 6025 B91 0.8 1.8 2 H Example 5 Comparative 1.68 0.084 4200 C to B 91 0.8 1.8 2H Example 6 Comparative 1.56 0.083 4160 C to B 91 0.8 1.8 2 H Example 7Comparative 1.46 0.056 2760 C 91 0.8 1.6 3 H Example 8 Comparative 1.360.019 950 C 91 0.8 1.0 3 H Example 9 Comparative 1.56 0.082 4112 C to B91 0.8 6.3 2 H Example 10 Comparative 1.55 0.096 4800 B 91 0.8 1.6 2 HExample 11 Comparative 1.55 0.083 4137 C to B 91 0.8 1.4 2 H Example 12Comparative 1.50 0.109 5463 A 91 0.8 1.6 2 H Example 13

INDUSTRIAL APPLICABILITY

While maintaining its suitability in mass production, the foldabledisplay using the polyester film or the hard coating film for foldabledisplays of the present invention is unlikely to deform after thepolyester film or the hard coating film positioned on the front surfaceof the foldable display is repeatedly folded, thus not causing imagedistortion at the folding portion of the display. In particular, amobile device or image display device equipped with the foldable displayusing the polyester film or hard coating film of the present inventionas a surface protection film provides beautiful images and has a varietyof functionality, while being highly convenient such as in portability.

DESCRIPTION OF THE REFERENCE NUMERALS

-   1: foldable display-   11: bend radius-   2: polyester film for surface protection films of foldable displays-   21: folding portion-   22: bending direction (the direction orthogonal to the folding    portion)

1. A foldable display comprising a surface protection film and apolarizer, wherein the surface protection film comprises a polyesterfilm, the angle made by the slow axis of the polyester film with theabsorption axis of the polarizer is 10 to 80°, and the polyester filmsatisfies the following conditions: (1) the polyester film has arefractive index in the bending direction of 1.590 to 1.620, (2) thepolyester film has a refractive index in the direction of a foldingportion of 1.670 to 1.700, (3) the polyester film has a refractive indexin the thickness direction of 1.520 or less, and (4) the polyester filmhas a density of 1.380 g/cm³ or more, wherein the bending directionrefers to a direction orthogonal to the folding portion of the polyesterfilm to be folded.
 2. The foldable display according to claim 1, whereinthe polyester film has an in-plane retardation (Re) of 3000 to 30000 nm.3. The foldable display according to claim 1, wherein the polyester filmhas a total light transmittance of 85% or more, a haze of 3% or less,and a maximum heat shrinkage of 6% or less.
 4. The foldable displayaccording to claim 1, comprising an easy-to-adhere layer on at least onesurface of the polyester film.
 5. The foldable display according toclaim 1, comprising a hard coating layer having a thickness of 1 to 50μm on at least one surface of the polyester film.
 6. A mobile devicecomprising the foldable display of claim
 1. 7. The foldable displayaccording to claim 2, wherein the polyester film has a total lighttransmittance of 85% or more, a haze of 3% or less, and a maximum heatshrinkage of 6% or less.
 8. The foldable display according to claim 7,comprising an easy-to-adhere layer on at least one surface of thepolyester film.
 9. The foldable display according to claim 8, comprisinga hard coating layer having a thickness of 1 to 50 μm on at least onesurface of the polyester film.
 10. A mobile device comprising thefoldable display of claim 9.